GP's passive is arguably one of the worst passives in the game.
You see, champions that revolve around their passives have not that awesome kits (Vel'Koz, Lucian, Tristana, etc) and champions that usually don't care about their passives have powerful kits (Lissandra, Lee Sin, Kog'Maw, Soraka, etc). Obviously, there's champion with good passives and good kits, but that's another different situation and not a problem at all, when balanced.
Then, you have GP. Not a powerful kit, neither a powerful passive. His passive revolves around sustained damage and kiting, but it's the only passive in the game that is affected by both defensive stats and tenacity.
This passive is only useful at all if you are going for DPS GP (which will likely fall off a bit in the late game) or for early trading. It serves no purpose mid/late game, since the slow is negligible and the damage is terrible.
Suggestion: make his passive true damage, with lower values in the early game and decent values in the late game.
> Gangplank's basic attacks apply a poison debuff for 3 seconds that can stack up to 3 times. Each stack deals 3 + (1 × level) magic damage per second and slows the target by 7%.
> Gangplank's basic attacks and Parrrley (I know it's already like that, just to clarify) applies a poison debuff for 3 seconds that can stack up to 3 times. Each stack deals 2 + (+ Level)(+0.05 BONUS AD) true damage per second and slows the target by 6%/7%/8%. Damage is not reduced by tenacity effects.
Not OP at all and more useful. Damage stays around the same in the early game with the diminished values.